The present invention relates to ceramic cutting tools having an integral chip control surface thereon. It especially relates to those ceramic cutting tools of the indexable type which are useful in the high speed machining of metallic materials.
Although ceramic cutting tools have demonstrated significant speed and/or tool life advantages in machining ferrous and non-ferrous materials in relation to cemented carbide, coated cemented carbide and cermet cutting tools in a number of applications, their usefulness remains limited by the commercial unavailability of inserts with positive rake molded chip control designs. Despite references to these designs in the literature (see U.S. Pat. Nos. 4,259,033; 4,340,324; 4,318,645; 4,087,193; 4,247,232), it is generally believed by those of ordinary skill in the art that such molded chip control designs in ceramic inserts will cause premature failure of the cutting edge during cutting operations. This belief is based on the lower transverse rupture strength and fracture toughness of ceramic cutting tool materials compared with cemented carbide and cermet cutting tools.
Compounds have been added to ceramics to increase their fracture toughness and transverse rupture strength. Such compounds, as silicon carbide whiskers, and titanium carbide, generally make the resulting composite more difficult to fabricate, insofar as a higher sintering temperature or hot pressing is required to achieve the full density needed to obtain the maximum fracture toughness and transverse rupture strength. It should be noted that, even when fully dense, the fracture toughness and transverse rupture strength of these ceramic composites are still well below those of cermets and cemented carbides.
These higher fabrication temperatures also lead to increased formation of a reaction layer at the surface of the ceramic composite. These reaction layers have a toughness and transverse rupture strength which is lower than that of the bulk material. Thus, in areas where it is critical to cutting performance that these surfaces have optimum strength and fracture toughness, these reaction layers have been ground off. These grinding requirements, therefore, make the fabrication of ceramic cutting inserts with chip control structures expensive and, where complex chip control structures are required, commercially impractical.
However, in most cases, in order to commercially and practically apply ceramic cutting inserts to the automatic (i.e., unmanned) high speed machining of ductile material, such as soft carbon, alloy and stainless steels and ductile or malleable cast irons, which have a tendency to form undesirably long chips during high speed machining, some form of chip control is needed to provide the desired short chips.
In the past, a separate, nonintegral chipbreaker was clamped to the flat top rake face of ceramic inserts to provide a degree of chip control, where necessary (see "A Practical Guide to Carbide," PL Technical Services, Sidney, Ohio (1981) Pages 28-33). However, at low feeds, less than about 0.008 inches, most chipbreaker plates are difficult to properly position, resulting in poor chip control in soft steel machining.
Another attempted solution in the prior art was to provide in the top rake surface of the ceramic insert an integral rising chipbreaker structure (i.e., a shelf type chipbreaker). U.S. Pat. No. 4,616,963 shows such a prior art ceramic cutting insert. A bevel (T-land or K-land) is provided on the rake face adjacent the cutting edge. An island is provided on the rake face having a molded concave wall rising from and above the bevel. Both the bevel and flank face are in a ground condition. The advantage of this design is that it retains the strong cutting edge (i.e., the included angle formed by the bevel and the flank face at the cutting edge is greater than 90 degrees) of the ceramic inserts with a flat rake face, while in some limited conditions providing chip control. (Walter W. Gruss, "Turning of Steel with Ceramic Cutting Tools," Tool Materials for High-Speed Machining, ASM International (1987) Pages 105-115, see FIG. 11.) Unfortunately, is design and the separate top clamp design tend to crowd, or impede the flow of, the chip as it is formed, and it is thereby believed to increase the power required to cut while also increasing the stresses applied by the chip at or near the cutting edge, leading to reduced cutting edge lifetime. The grinding of the bevel and flank face are also costly operations.
Therefore, there has long been a need in the art for ceramic cutting inserts having a chip control structure, preferably in a positive rake design, and which can be economically and readily manufactured, while providing chip control and acceptable metalcutting lifetimes at high metalcutting speeds over a range of feeds and depths of cut.